1. Field of the Invention
This invention relates to control systems for spectroscopic devices, and more particularly, to a demand modulated atomization system particularly adapted for use in plasma spectroscopy.
2. Background Information
In atomic absorption spectroscopy, atomic fluorescence spectroscopy, and atomic emission spectroscopy, a means must be provided to atomize a source of sample material so that the sample's absorption, fluorescence, or emission spectra may be observed. In the past, atomization has been accomplished by using a nebulizer or an electrothermal atomizer. Various aspects of the work in the area of electrothermal atomization have been patented. (See, for example, U.S. Pat. No. 4,407,582 to Woodriff, issued Oct. 4, 1983, and U.S. Pat. No. 4,529,307 to Holcomb, et al., issued Jul. 16, 1985).
During the 1970's, a new analytical technique known as inductively coupled plasma (ICP) emerged. A plasma is defined as a luminous gas, a significant fraction of whose atoms or molecules are ionized. Plasmas therefore are considered to be gaseous conductors. As such, plasmas readily interact with magnetic fields, making it possible to couple a plasma to a high frequency power source. The emergence of ICP techniques led to the wide spread use of ICP atomic emission spectroscopy systems. ICP atomic fluorescence spectroscopy systems have also been developed. (See for example, U.S. Pat. No. 4,300,834 to Demers, et al., issued Nov. 17, 1981).
Even with use of an ICP technique, some means must be used to introduce analyte (that is, the material to be analyzed) to the ICP system (commonly known as a "torch"). One such means is a nebulizer, which introduces a mist of liquid or dissolved analyte to the ICP torch. This has the disadvantage of limiting the types and concentrations of analyte that may be examined.
An electrothermal atomization device may also be used to provide atomized samples of test material for injection into an ICP torch. In the past, all such systems known to the inventor produce transient signals dependent on conditions pre-set by an operator. This requires a knowledge of the contents of the sample being tested and is not compatible with high volume production work where such prior knowledge is not practical, and where concentrations of various elements may vary over many orders of magnitude. Furthermore, presently existing ICP spectroscopy instruments provide poor control of the rate of production of ions of any and all elements present in a sample, and are frequently limited in the types of materials that can be used as a sample. For example, many such ICP spectroscopic instruments are unable to test samples comprising solid material, or solutions containing high amounts of dissolved solids. In other prior art instruments, such samples can be tested only by reconfiguring the entire instrument, which can be a time-consuming task if precise measurements are to be obtained.
In examining the prior art, it has been discovered that a major problem in obtaining adequate results in commercially viable instruments has been an inability to accurately control the atomization rate of sample material being injected into an ICP torch. This is especially true in the recently developed technique known as ICP/Mass spectroscopy. It is therefore highly desirable that some means or method be developed that accurately controls the analyte atomization rate.